This application is a U.S. national phase of International Application No. PCT/F197/00623, filed Oct. 14, 1997.
The present invention relates to a method and apparatus for heating pulps. In particular, the method and apparatus in accordance with the invention are applicable to heating medium-consistency fiber suspensions of the wood processing industry with low-pressure steam.
In wood processing industry, it is frequently necessary to heat or cool consistent pulp suspensions at a consistency range of 6-20%. Not until in the middle of the 1980""s was it possible to do this economically with either direct heating or by means of an indirect heat exchanger. To begin with, some examples are now used to illustrate how pulp is heated or cooled by means of the present-day technology in the mill scale.
In connection with bleaching stages, for example, it is often necessary to raise the temperature of the pulp by 10-20xc2x0 C., occasionally even by 30xc2x0 C., in order to achieve the right reaction temperature. The heat is usually raised in such a way that steam is mixed into the pulp prior to the pumping. The mixing is effected either by a peg mixer, which is a large-sized, heavy and expensive device consuming a great deal of energy, or by steam injectors for example into a drop leg for the pulp upstream of a pulp pump. This technique has certain disadvantages, one of them being the noise resulting from this kind of direct heating. Another disadvantage is that because of the large volume of steam, it is not possible to mix very large amounts of steam into the pulp. Yet a third disadvantage is that the pulp becomes heated unevenly, because the heating is, in practice, always performed in open unpressurized apparatus, in which the condensation of the steam is unreliable and uneven. When using unpressurized mixing techniques, the heating may be performed by low-pressure steam, which, although being a very economical source of heat, results in the use of large-sized apparatus. Furthermore, it is self-evident that when using low-pressure steam the upper limit of the temperature will be about 90-95xc2x0 C. under unpressurized conditions. Thus, due to above-described disadvantages, the temperature can only be raised to a certain extent, in practice approximately by 10-15xc2x0 C. at the maximum. Of course, it is possible to raise the temperature even by 20xc2x0 C., but in that case, the apparatus used will be, virtually speaking, unreasonably large. To avoid above-described disadvantages and to make the heating of pulps more efficient, the development of an indirect heating method was set about at the latter half of the 1980""s.
Indirect heat exchangers of this type, i.e. so called MC heat exchangers are described in for example EP patent 275502, FI patent applications 781789, 943001, 945783, 953064, 954185 and 955007 and international patent application PCT/FI96/00330. These numerous applications are based on the fact that consistent pulp forms a strong fiber network at a consistency range of 6-20% whereby dividing or combining pulp in flow channels is not possible without special measures. As the consistent pulp reaches a breaching point, the fiber network may be so strong that the pulp flow will not be able to divide by itself. Possibly, the fiber network will stick to uneven points in the flow channel, which results in discharging of water and clogging. Also, combining two flows is difficult. The internal forces of the fiber network are so powerful that two smaller flows will not be able to form a larger, uniform flow without special measures. Required measures being taken, the technical realization of the apparatus becomes possible and the low-pressure steam is used as the source of heat. On the other hand, the apparatus is, at least for the time being, relatively expensive and difficult to manufacture, and therefore an indirect heat exchanger in heating consistent pulps can be applied to only a few, selected objects of use. Thus, the development of an indirect heat exchanger is still at such a phase that there are also grounds to reflect upon the use of direct low-pressure steam in heating pulp.
Thus, it would nevertheless be preferable to use low-pressure steam for direct heating of pulp. In cellulose pulp mills, low pressure steam is classified as waste, the removal of which, i.e. the condensation, has to be arranged in one way or another. If the amount of heat in the low-pressure steam could be utilized in mill processes, it would be possible to sell a larger part of the energy produced at the mill.
However, above-described prior art heating methods based on the use of low pressure steam have turned out to be unreliable. According to our observations, one reason is that when supplying the steam into an atmospheric drop leg from the bottom of which the pulp is removed by pumping, the steam tends to rise in the direction of the lower pressure, i.e. upward, in other words away from the pump. Hence, part of the steam discharges from the pulp, whereby it is virtually necessary to restrict the supply of steam into pulp to such an amount that the condensation of the steam into the pulp is ensured. Using this method, the temperature cannot be raised more than approximately 10 degrees at most. Naturally, one solution, which is even used to some extent within the industry, would be to supply the steam at a high pressure from the drop leg into the pulp to be removed to the pressurized side of the discharge pump, whereby the steam would not have a possibility to discharge anywhere else from the pulp but the only option would be the heating of the pulp by as many degrees as would be required by the amount of the heat in the steam. However, high pressure steam is expensive to use, and therefore it would be highly preferable to avoid the use thereof. SE patent 412610, FI patent application 951196 and SE patent application 9501094 disclose an apparatus enabling the use of low-pressure steam in direct heating of pulp in such a way that the pulp to be heated is made flow in the pipe system from one process stage to another by a pump raising the pressure of the pulp by only a few bar, leaving, however, the pressure of the pipe system lower than the pressure of the steam used for the heating. The steam is mixed into such flowing pulp by means of a special mixer, which is either a rotating mixer described in for example SE patent 419 603, or by means of a basically static mixer described in WO patent application 95/21016. Thereafter, the pulp flows to a second pump, by means of which the pressure of the pulp is raised to a sufficient value for the following process stage, in which the pulp is introduced into an atmospheric or pressurized reaction vessel. In the methods according to the above-described publications, it is, however, considered necessary to mix the steam by means of a special fluidizing or at least efficiently mixing apparatus. A more conventional mixer disclosed in SE-B-419 603 mentioned by said SE application 9501094 is a fluidizing mixer originally intended for mixing oxygen, chlorine and chlorine dioxide into the fluidized pulp in the apparatus. The capacity required by such an MC mixer is also very high. Moreover, the rotor of a fluidizing MC mixer rotates axially relative to the flow, whereby a vortex is formed (induced) on the inlet side of the mixer. In practice, this means that the pulp suspension has a component v0 parallel to the tangent of the rotor already when arriving in the mixing area. Thus, as the rotor rotates at a velocity v1, the pulp only has to speed up by the amount v1-v0. The intensity of the turbulence, i.e. the mixing efficiency, would be higher if v0 was zero, which is what is aimed at in the inlet conduit of the mixing chamber in accordance with our invention. Another, much more recent publication WO 95/21016 describes a basically static mixer, in which the pulp flowing in the flow pipe is forced to flow through a very narrow slot, whereby the velocity of the pulp naturally increases in relation to the flowing surfaces. In other words, the flow velocity of the pulp in the slot is in practice multiple compared with the flow in the pipe, even so great that the pulp may be considered to be fluidized in the slot, into which the chemical or steam to be mixed is introduced. In other words, both of said apparatus alternatives are characterized by the fact that the pulp is subjected to a mechanical effect in a separate mixer in order to change the state thereof, so that the steam can be mixed evenly into the pulp.
The disadvantages of such direct steam heating apparatus are that, firstly, three separate means are required, i.e. a pump, a mixer, and a second pump; secondly, the mechanical properties of the pulp change in each treatment, whereby the pulp strength deteriorates to some extent; and thirdly, a certain pressure loss always takes place in prior art mixers.
Now we have observed that as regards the overall economy of a mill, the most preferable way to heat pulp would be to effect the heating by direct low-pressure steam in an apparatus comprising at least one pump, one steam mixing means raising the pressure and one feeding means for low-pressure steam between them. In other words, when allowing the pulp to flow through a steam feeding means as an even plug flow, the pulp is not subjected to any kind of stress. The feeding means is positioned into the suction pipe of a mixing means raising the pressure at a desired distance from the mixing means.
The characterizing features of the present method and apparatus become apparent from the appended claims.